Re: [PATCH 2/2] x86/hw_breakpoint: Add arch_reinstall_hw_breakpoint

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 10 Sep 2025 17:39:35 +0800
Jinchao Wang <wangjinchao600@gmail.com> wrote:

> The new `arch_reinstall_hw_breakpoint` function can be used in an
> atomic context, unlike the more expensive free and re-allocation path.
> 
> This patch adds `BP_SLOT_ACTION_REINSTALL` to the `enum` and updates the
> `manage_bp_slot` helper to correctly handle the action. This allows
> callers to efficiently re-establish an existing breakpoint.
> 

Ah, this also looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks,

> Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
> ---
>  arch/x86/include/asm/hw_breakpoint.h |  2 ++
>  arch/x86/kernel/hw_breakpoint.c      | 21 +++++++++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
> index bd437a30dbf2..d1cc6db8a59f 100644
> --- a/arch/x86/include/asm/hw_breakpoint.h
> +++ b/arch/x86/include/asm/hw_breakpoint.h
> @@ -20,6 +20,7 @@ struct arch_hw_breakpoint {
>  
>  enum bp_slot_action {
>  	BP_SLOT_ACTION_INSTALL,
> +	BP_SLOT_ACTION_REINSTALL,
>  	BP_SLOT_ACTION_UNINSTALL,
>  };
>  
> @@ -64,6 +65,7 @@ extern int hw_breakpoint_exceptions_notify(struct notifier_block *unused,
>  
>  
>  int arch_install_hw_breakpoint(struct perf_event *bp);
> +int arch_reinstall_hw_breakpoint(struct perf_event *bp);
>  void arch_uninstall_hw_breakpoint(struct perf_event *bp);
>  void hw_breakpoint_pmu_read(struct perf_event *bp);
>  void hw_breakpoint_pmu_unthrottle(struct perf_event *bp);
> diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
> index 1736063a82b7..99223695cee8 100644
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -94,6 +94,10 @@ static int manage_bp_slot(struct perf_event *bp, enum bp_slot_action action)
>  		old_bp = NULL;
>  		new_bp = bp;
>  		break;
> +	case BP_SLOT_ACTION_REINSTALL:
> +		old_bp = bp;
> +		new_bp = bp;
> +		break;
>  	case BP_SLOT_ACTION_UNINSTALL:
>  		old_bp = bp;
>  		new_bp = NULL;
> @@ -192,6 +196,23 @@ int arch_install_hw_breakpoint(struct perf_event *bp)
>  	return arch_manage_bp(bp, BP_SLOT_ACTION_INSTALL);
>  }
>  
> +/*
> + * Reinstall a hardware breakpoint on the current CPU.
> + *
> + * This function is used to re-establish a perf counter hardware breakpoint.
> + * It finds the debug address register slot previously allocated for the
> + * breakpoint and re-enables it by writing the address to the debug register
> + * and setting the corresponding bits in the debug control register (DR7).
> + *
> + * It is expected that the breakpoint's event context lock is already held
> + * and interrupts are disabled, ensuring atomicity and safety from other
> + * event handlers.
> + */
> +int arch_reinstall_hw_breakpoint(struct perf_event *bp)
> +{
> +	return arch_manage_bp(bp, BP_SLOT_ACTION_REINSTALL);
> +}
> +
>  /*
>   * Uninstall the breakpoint contained in the given counter.
>   *
> -- 
> 2.43.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>